Overvoltage protection circuit for a generator regulator system

ABSTRACT

An overvoltage protection circuit for a vehicle electrical generating system provides a plurality of controlled rectifiers connected to the generator output winding for shunting high transient voltages to ground. A control transistor having a time delay biasing circuit is connected to the field winding excitation circuit for holding the field winding deenergized during the high voltage condition. A single overvoltage detecting device is used to render both the plurality of controlled rectifiers and the control transistor conductive.

United States Patent Inventors Thomas E. Kirk;

Gerald II. Stenklyft, both of Anderson, Ind. 26,707

Apr. 8, 1970 May 25, 1971 General Motors Corporation Detroit, Mich.

Appl. No. Filed Patented Assignee OVERVOLTAGE PROTECTION CIRCUIT FOR AGENERATOR REGULATOR SYSTEM 2 Claims, 1 Drawing Fig.

U.S. Cl 317/16, 317/31, 317/50, 320/64, 322/28 Int. Cl H02h 3/22, H02j7/14 Field of Search 320/61, 64,

SCR; 322/28; 317/13, 16, 31, 50

[56] References Cited UNITED STATES PATENTS 3,488,560 1/1970 Konopa320/SC 3,491,285 [/1970 Nowakowski 322/28x Primary Examiner-J. D. MillerAssistant Examinerl-larvey Fendelman AttorneysE. W. Christen, C. R.Meland and Robert W.

Smith ABSTRACT: An overvoltage protection circuit for a vehicleelectrical generating system provides a plurality of controlledrectifiers connected to the generator output winding for shunting hightransient voltages to ground. A control transistor having a time delaybiasing circuit is connected to the field winding excitation circuit forholding the field winding deenergized during the high voltage.condition. A single overvoltage detecting device is used to render boththe plurality of controlled rectifiers and the control transistorconductive.

OVERVOLTAGE PROTECTION CIRCUIT FOR A GENERATOR REGULATOR SYSTEM Thisinvention relates to vehicle electrical generating systems and moreparticularly to an overvoltage protection circuit for preventing hightransient voltages from being developed across the electrical componentsof a vehicle generating system.

In the electrical generating systems of motor vehicles, transistorregulating circuits are employed to control the voltage and currentoutputs of an engine driven generator which charges the vehicle batteryand supplies other electrical loads. It has been found that duringcertain operating conditions excessive voltages are developed in thesystem and in one specific instance, high transient voltages occur whenthe electrical loads are suddenly disconnected from the generatoroutput. This can occur, for example, by a loose connection at thegenerator output terminals or by inadvertent opening of a loaddisconnect switch with the system operating.

when the generator load changes abruptly to an open circuit condition,also referred to as a load-dumping situation, high energy is released atthe generator output causing an instantaneous voltage rise. Highpotential transient voltages are produced which are capable of damagingthe semiconductors and other electrical components connected, forexample, in the generator regulating circuits. It is therefore desirableto protect these components from damage, or, otherwise, more expensivecomponents having high voltage characteristics must be used.

It is further. desirable to assure that the field winding is maintaineddeenergized when the overvoltage condition occurs'so that-the reactivefield current will decay to a low value before being reenergized by thegenerator regulating circuits. Reactive field current normally flowswhen the inductive energy stored in the field winding dischargesfollowing turn off of field current. Due to the relative long timeconstant of the field winding circuit, the reactive current continuessubstantially longer than the high transient voltage. Accordingly, ifcurrent is supplied to field winding before the reactive current decays,further high voltage transients are developed at the generator outputwinding and the overvoltage condition will continue.

In accordance with this invention an overvoltage protection circuit isprovided-in a vehicle electrical generating system including atransistor voltage regulating circuit which controls the generator fieldcurrent in response to a desired generator output voltage. A shuntingcircuit is provided including controlled rectifiers connected betweenthe generator output winding terminals and a system ground connection. Acontrol transitor is provided for deenergizing the field winding and isconnected across the resistor-capacitor to the regulating circuit outputtransistor which is operative to switch the field current off and on. Anovervoltage sensing device provided by a zener diode is connected to thegate electrodes of the controlled rectifiers of the shunting circuit andto the biasing circuit of the control transistor. They are renderedconductive when thezener diode reaches a predetermined breakdown voltagecaused by an overvoltage condition developed in the generator outputcircuit. A resistor-capacitor time delay network is connected in thebiasing circuit of the control transistor so that the output transistoris held nonconductive for a predetermined time. Accordingly, the fieldwinding is maintained deenergized for a sufficient time to enable thereactive field current to become substantially fully discharged prior tothe regulating circuits regaining control of the genera- It is an objectof this invention to provide an improved vehicle electrical generatingsystem in which an overvoltage protection circuit prevents excessivelyhigh voltages from being developed across electrical components includedin the system by shunting the generator output to ground andconcurrently maintaining the field winding deenergized.

Another object of this invention is to provide a vehicle electricalgenerating system in which an overvoltage protection circuit is providedincluding a voltage breakdown sensing nected from the system, andfurther in which a shunting circuit is provided for bypassing the highvoltages developed at the output winding terminals to ground, and also,in which a control transistor biases a regulating circuit outputtransistor nonconductive so that the field winding is prevented frombeing energized until reactive field current is fully dissipated.

And a still further object of this invention is to provide anovervoltage protection circuit for a vehicle electrical generatingsystem including an alternating current generator having an outputwinding connected to a power rectifier circuit in which the overvoltageprotection circuit includes a plurality of controlled rectifiers whichform a shunting circuit connected between the output winding and agrounded terminal and in which a zener diode is connected between thepower rectifier circuit and the gating electrodes of the controlledrectifiers so that they are gated conductive when the zener diodeconducts in response to an overvoltage condition, and further in which acontrol transistor is connected to a regulating circuit outputtransistor and includes a resistor-capacitor time delay network whichbiases the control transistor conductive for a predetermined time afteran overvoltage condition develops so that the output transistor isbiased nonconductive until the reactive field current has becomesufficiently discharged prior to the generating system being returned toa normal operative condition.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawing wherein a preferred embodiment of the present invention isclearly shown.

In the drawing, a single FIGURE illustrates a schematic circuit diagramof an electrical generating system intended for use in motor vehicle andincluding an overvoltage protection circuit made in accordance with thisinvention.

Referring now to the drawing, a schematic circuit diagram illustrates anelectrical generating system I0 for supplying direct current toelectrical loads of a motor vehicle. The generating system 10 includesan alternating current generator 12 having a three-phase Wye-connectedoutput winding 14 and a field winding 16. The output winding 14 developsthreephase altematingcurrent in response to the driven speed of thegenerator shaft and the current through the field winding 16.

A power rectifier circuit 18 is connected to the generator outputwinding terminals 20, 22 and 24 for developing the direct current outputprovided by the generating system 10. The upper and lower circuitjunctions of the rectifier. circuit 18 form a positive power outputterminal 26 and a negative power output terminal 28, respectively. Theterminal 26 is also referred to as the generator output since therectifier circuit I8 is integrally mounted within the housing ofgenerator 12. The negative power output terminal 28 is connected to asystem reference point or potential indicated by numeral 30 which isprovided by grounding the system to the vehicle chas- SIS.

The vehicle electrical loads supplied by the electrical generatingsystem 10 are represented by a resistive load 32 and a vehicle battery34. A cable conductor 36 connects the positively poled terminals of theresistive load 32 and battery 34 to the power output terminal 26. Thenegatively poled terminals of the resistive load 32 and battery 34 areconnected to the system ground 30 provided at the vehicle chassis. Thevoltages developed at the power output terminal are typically l2 or 24volts DC.

The electrical generating system 10 further includes a field windingexcitation circuit 38, a transistor voltage regulating circuit 40, andan overvoltage protection circuit 42 illustrated within the box outlinedesignated 44. The field winding excitation circuit 38 connects thefield winding 16 across the output winding I4 and the power rectifierbridge circuit 18 by a series circuit connection provided across thepositive and negative power output terminals 26 and 28. A conductor 46connects the power output terminal 26 to the upper end of a resistor 48.The upper end, 49, of the field winding 16 is connected to the lower endof the resistor 48. A field discharge diode 51 is connected with theindicated polarity across both the field winding 16 and the resistor 48.

A pair of NPN transistors 50 and 52, which are connected in a Darlingtonamplifier circuit arrangement, provide an output transistor switchingdevice 54 of the voltage regulating circuit 40. The collector-emitteroutput circuit of the output transistor 54 is connected between thelower terminal 55 of the field winding 16 and a grounded conductor 56.The conductor 56 is connected to the ground potential 30 at junction 58to complete the excitation circuit 38 connection between the positiveand negative power terminals 26 and 28. The output transistor 54 isrendered conductive and nonconductive by the voltage regulating system40, as described further hereinbelow, to regulate the field currentsupplied to the field winding 16.

The overvoltage protection circuit 42 includes a shunting circuit 60comprising three controlled rectifiers 62, 64 and 66 of the siliconcontrolled rectifier (SCR) type having anode, cathode and gateelectrodes. The anodes of the three controlled rectifiers are connectedto the generator output terminals 20, 22 and 24, respectively, and thecathodes are connected together'at circuit junction 68 which isconnected to the ground 30. A zener diode 70 including a predeterminedreverse breakdown voltage characteristic provides an overvoltage sensingdevice. The cathode of the diode 70 is connected to the conductor 46 atjunction 71 and the anode electrode is connected to each of the gates ofthe three controlled rectifiers 62, 64 and 66.

The overvol tage protection circuit 42 further includes a field windingdeenergizing circuit portion 72 including a control transistor 74. Thecontrol transistor 74 is an NPN type having base, collector and emitterelectrodes. The collector is connected to a conductor 76 at junction 78which is connected to the base of the transistor 50 and provides aninput biasing connection to the output transistor 54. The emitter isconnected to the conductor 56 at the junction 80 so thecollector-emitter circuit is connected across the input biasing circuitof the output transistor 54. I

A time delay network comprising a resistor 86 and capacitor 88 isconnected across the base-emitter circuit of the control transistor 74.The base is connected to one end of the resistor 86 and the other end isconnected to junction 90 formed between the resistor 86 and upper end ofthe capacitor 88. The lower end of the capacitor 88 is connected to thejunction 92 of the conductor 56 and thereby to the emitter of thecontrol transistor 74.

A single controlled rectifier 94 of the silicon controlled rectifier(SCR) type having cathode, anode and gate electrodes is included in thefield deenergizing circuit 72 for charging the capacitor 88 and biasingthe control transistor 74 conductive. The cathode of the controlledrectifier 94 is connected through the conductor 96 to the junction 90and the anode is connected to the conductor 46 at junction 98. The gateof the controlled rectifier 94 is connected through a coupling capacitor100 to the anode of the zener diode 70 at junction 102. The controlledrectifier 94 and the controlled rectifiers 62, 64 and 66 of the shuntingcircuit 60 are gated conductive when the zener diode 70 conducts reversebreakdown voltage in response to a predetermined overvoltage potentialdeveloped between the positive and negative power terminals 26 and 28,as described more fully hereinbelow.

Referring now more specifically to the remaining detail portions of thegenerating system 10, the power rectifier circuit 18 includes sixsilicon power diodes 108 connected in a conventional three phasefull-wave rectifier bridge arrangement with three upper diodes of thediodes 108 poled in a positive direction relative to the output windingterminals 20, 22 and 24 respectively. The cathodes of these three diodesare connected together at the power output terminal 26. The lower threeof the six diodes 108 are poled in a negative direction relative to theoutput winding terminals 20, 22 and 24 and the anodes of these threediodes are connected together at the negative power terminal 28. Adesired direct current output regulated at 24 volts, for example, isprovided across the positive and negative power output terminals 26 and28 in accordance with the arrangement and operation of the voltageregulating circuit 40 as described hereinafter.

The voltage regulating circuit 40 generally includes the outputtransistor 54 and a voltage responsive sensing arrangement including avoltage divider network 128, a voltage reference circuit and a drivertransistor 132. The voltage network 128 is connected to the generatingsystem 10 by a conductor 134 which is connected to the junction 138between the resistor 48 and upper terminal 49 of the field winding 16.The potential developed across the conductor 134 and ground provides avoltage sensing signal which varies in response to changes in thevoltage developed across the positive and negative power outputterminals 26 and 28. The resistor 48,-so-called a pickup resistor,varies the potential developed at junction 138 and on conductor 134 inresponse to increase and decrease of generator current. This is toincrease or pick up" the voltage sensing signal to increase the voltageprovided across the loads 32 and particularly the battery 34 and therebycompensate for voltage drop occurring across the cable conductor 36 atincreased generator current levels.

An auxiliary rectifier circuit 140 including three silicon power diodes142 provides a source of direct current from the output winding 14 tothe voltage regulating system 40. The three diodes 142 include cathodeelectrodes which are respectively connected to the output windingterminals 20, 22 and 24 and anode electrodes which are connectedtogether by the conductor 144. The auxiliary rectifier circuit 140 formsa fullwave rectifier bridge with the lower three of the diodes 108 toproduce a positive voltage on the conductor 144. A system turn off ordisconnect feature is also provided by the auxiliary rectifier circuit140 to block current flow from the battery 34 and through the voltageregulating circuit 40 or power rectifier circuit 18 when the system isnot in use.

The voltage divider network 128 is formed by a series circuit includinga fixed resistor 150, an NPN transistor 152 having collector, emitterand base electrodes, and a variable resistor 154. The upper end of theresistor is connected to the conductor 134 to connect the voltagedivider network 128 to the voltage sensing signal and the lower end ofthe resistor 150 is connected to the collector of the transistor 152.The emitter of the transistor 152 is connected to the upper end of thevariable resistor 154 and the lower end of this resistor is connected tothe conductor 56. The base of the transistor 152 is connected through aresistor 158 to the junction 160 on the conductor 144. The voltagedeveloped at junction 160 forward biases the base-emitter circuit of thetransistor 152 to render the collector-emitter circuit of the transistorconductive when voltage is developed in the output winding 14. Thisoperatively connects the voltage divider network 128 across theconductors 134 and 56. The output of the voltage divider network 128 isprovided at the junction 162 on the slider arm of the variable resistor154. The junction 162 is connected to a capacitor 164 and a zener diode166 at the junction 168. The capacitor 164 is further connected tojunction 170 of the conductor 56 to filter fluctuating voltagesdeveloped across the output of the voltage divider network 128.

The voltage reference circuit 130 includes the zener diode 166 having acathode electrode connected to the output of the voltage divider networkat the junction 168. The anode electrode is connected to a silicon diode172 which is poled in series opposition to the zener diode 166. The baseof the driver transitor 132 is connected to the cathode electrode of thediode 172. The zener diode 166 has a predetermined reverse voltagebreakdown characteristic intended to bias the baseemitter circuit of thetransistor 132 conductive in accordance with the desired regulatedvoltage which is developed across the power terminals 26 and 28. Thediode 172 is provided to compensate for temperature variations in thereverse voltage breakdown characteristics of the zener diode I66 wherebya substantially constant voltage response is provided regardless ofchanges in ambient temperature.

Referring further to the driver transistor 132, the emitter is connectedto the conductor 56 at junction 58 and the collector is connectedthrough a resistor [80 to conductor 182 which is connected to a junction184 on the conductor 144. The collector is also connected at junction186 to the conductor 76 connected to the base of the transitor 50 andthe collector of the transistor 74, as noted hereinabove. The voltageoccurring at the collector of the transistor 132 and at conductor 76 isdeveloped by the positive voltage occurring at the junction 184 at theoutput of the auxiliary rectifier bridge 140. Ac-

cordingly, the voltage of the conductor 76 has a first predeterminedvalue which is a positive level when the driver transistor 132 isnonconductive. The voltage of conductor 76 has a second predeterminedvalue which is at a substantially lower level, approximately equal toground potential at the conductor 56, when the collector-emitter circuitof the driver transistor 132 is rendered conductive.

The driver transistor 132 is normally nonconductive when the voltage atthe junction 162 provides a voltage divider network output voltage belowthe voltage required to cause reverse voltage breakdown current flow inthe zener diode 166. This voltage corresponds to 'a voltage beingdeveloped across the positive and negative power output terminals 26 and28 which is below the desired regulated value. Accordingly, voltage atjunction 186 is provided at the first predetermined value.

The driver transistor 132 is biased conductive when the zener diode 166conducts when its reverse breakdown voltage characteristic is exceededto provide forward biasing drive current through the base-emittercircuit of the transistor 132. The collector-emitter circuit of thetransistor is rendered fully conductive so that voltage at junction 186is provided at the second predetermined value. Accordingly, the drivertransistor 132 and the voltage reference circuit 130 provide a voltageresponsive means that is responsive to the voltage developed at theoutput of the voltage divider network 128 which in turn is proportionalto the voltage developed across the positive and negative power outputterminals 26 and 28.

The output transistor 54 of the voltage regulating circuit 40 is formedby the pair of NPN transistors 50 and 52 connected in the Darlingtonamplifier circuit arrangement as noted hereinabove. The coactingoperation of the Darlington amplifier transistors 50 and 52 provides aswitching function corresponding to that of a single switchingtransistor, thyristor or other gate-controlled semiconductor switchingdevice. The collectors of the transistors 50 and 52 are connectedtogether and to the lower terminal 55 of the field winding 16. Thebaseemitter circuits of the two transistors are connected in seriesbetween the conductor 76 and the conductor 56. Accordingly, both arebiased conductive and nonconductive in a switching mode of operation inresponse to the first and second predetermined voltage values,respectively, which are developed on the conductor 76. The normalswitching operation occurs at a relatively high rate so that theemitter-collector output circuit of the output transistor 54 switchesthe field current on and off rapidly to produce an average value ofcurrent flow which develops the desired regulated voltage across thepositive and negative power output terminals 26 and 28.

When the voltage regulating circuit 40 renders the output transistor 52nonconductive, the field winding excitation circuit 38 is deenergizedfrom across the positive and negative power output terminals 26 and 28.Reactive field current flow is developed which discharges the storedinductive energy in the field winding 16. A counter inductive voltagedevelops to oppose the change in field current flow and causes thereactive field current to flow in the forward direction of the diode 51.The reactive field current is discharged through the diode 51 and theresistor 48 and decays as the stored energy is dissipated in heat.

Referring now to the operation of the overvoltage current protectioncircuit 42, when a predetermined level of overvoltage is developed bythe output winding 14, as by high positive going transient voltagesdeveloped at terminal 26 when the generator load is suddenlydisconnected, the critical voltage breakdown level of the zener diode70, for example 50 volts, is reached so that is conducts current in thereverse direction. The high transient voltages developed at thegenerator output terminals 20, 22 and 24 are applied across the positiveand negative power output terminals 26 and 28 and to the conductor 46.The current through the zener diode 70 from the conductor 46 suppliesgating current to the controlled rectifiers 62, 64 and 66 so that theyare rendered conductive. This shunts the generator output windingterminals 20, 22 and 24 through the low impedance path provided by theanode to cathode circuits of the controlled rectifiers to ground 30 atthe junction 68.

At this time, the voltage drop developed across the output windingterminals 20, 22 and 24 and the power output terminals 26 and 28 will besubstantially equal to the small voltage drop which develops across theconducting anode to cathode circuits of the controlled rectifiers 62, 64and 66. This voltage is insufficient to maintain the zener diode 70conductive and the gating current to the controlled rectifiers 62, 64and 66 is interrupted. The alternating voltage occurring at theterminals 20, 22 and 24 is applied across the controlled rectifiers sothey are reverse biased during the negative voltage cycles and arereturned to a nonconductive state. The shunting circuit 60 will normallybypass the high transient voltages initially developed in the generatoroutput winding 14 in a relatively shorter time then the time required todissipate the reactive current in the field winding 16.

While the shunting circuit 60 is operative, the voltage developed on theconductor 46 drops to a low value corresponding to the voltage at thepower output terminal 26 thus preventing the high transient voltagesfrom being applied across the electrical components of the voltageregulating circuit 40. The low voltage developed on the conductor 46causes thevoltage regulating circuit 40 to be in an operative conditiontending to energize the field winding 16 before the reactive fieldcurrent has become discharged. Accordingly, the field windingdeenergizing circuit 72 is provided to maintain the output transistor 54nonconductive for a predetermined time which is in the range of 50 tomilliseconds. Otherwise, the output transistor 54 will be biasedconductive to supply current to the field winding 16 which will add tothe reactive field current. The current in the field winding 16 willthen be effective to produce a reoccurrence of the overvoltage conditionat the power output terminal 26. The controlled rectifiers 62, 64 and 66would continue oscillating between conductive and nonconductive statesand the overvoltage condition would continue.

The field winding deenergizing circuit 72 is rendered operative by thereverse voltage breakdown of the zener diode 70 at the same time thecontrolled rectifiers 62, 64 and 66 are rendered conductive. The hightransient voltage which causes the zener diode 70 to conduct will alsoproduce a triggering pulse through the coupling capacitor 100 to renderthe controlled rectifier 94 conductive. Following the conduction of thecontrolled rectifier 94, the capacitor 100 blocks current flow in thegating circuit of the controlled rectifier 94 so that it is isolatedfrom the gating circuits of the controlled rectifiers 62, 64 and 66. Ifthe capacitor 70 were not provided the conductor 96 would be preventedfrom reaching the potential of the conductor 46 developed by the hightransient voltage condition due to a series circuit path including thecathode to gate circuit of the controlled rectifier 94 and the gate tocathode circuits the controlled rectifiers 62, 64 and 66 which aregrounded at junction 68. The cathode to gate circuits would limit thevoltage of the conductor 96 to a range of approximately seven to eightvolts.

Accordingly, when the zener diode 70 applies a triggering pulse throughthe capacitor 100 to gate controlled rectifier 94 conductive thepotential of the conductor 46 is connected to the conductor 96. Thispotential will be approximately 50 volts as determined by the reversebreakdown voltage characteristic of the zener diode 70. The capacitor 88is charged from conductor 96 to a voltage of approximately 50 volts sothat the base-emitter circuit of the control transistor 74 is forwardbiased. As the capacitor 88 is charged, the voltage developed at thejunction 90 will raise the voltage at the cathode of the controlledrectifier 94 relative to the anode so that it is shut off since it isnot sufficiently biased in the forward direction to sustain conduction.

The voltage rise at junction 90 also provides base to emitter drivecurrent for forward biasing the transistor 74 to conduction. Thecapacitor 88 begins discharging through the resistor 86 when thecontrolled rectifier 94 shuts off. The time constant provided by thecapacitor 88 and a resistor 86 is such that forward biasing drivecurrent is maintained for a minimum of 50 to 100 milliseconds.Therefore, the time constant corresponds to the time interval requiredfor the reactive field current to be discharged from field winding 16and through the field discharge diode 51.

With the control transistor 74 being conductive, the potential of theconductor 76 is lowered to substantially the ground potential of theconductor 56 since there is negligible voltage drop across theconductive collector-emitter circuit of the control transistor 74. Thisprovides the above-mentioned second predetermined voltage value on theconductor 76 so that the output transistor 54 is prevented from beingbiased conductive by the voltage regulating circuit 40. Accordingly, thefield winding 16 cannot be energized from the field winding excitationcircuit 38 until after the time provided for dissipating the reactivefield current. The reactive fieldcurrent will decay to a level such thatadditional field current will not produce an overvoltage potential atthe positive and negative power output terminals 26 and 28 with thegenerator load disconnected.

Thus, the time delay network including resistor 86 and capacitor 88assures that the field winding reactive current decays to a sufficientlylow value so that the overvoltage potentials will not be redeveloped atthe generator output. After the field winding 16 is discharged, thevoltage regulating circuit 40 will be capable of regulating thegenerator outputso that further overvoltages do not occur with thegenerator load circuit being open-circuited.

While the embodiment of the present invention as herein disclosedconstitutes a preferred form, it is understood that other forms may beadopted within the spirit of this invention.

What We claim is as follows:

1. A motor vehicle electrical generating system for supplying directcurrent power at a voltage having a desired regulated value relative toa reference potential, said system comprising: an alternating currentgenerator including an output winding and a field winding which producesvoltage in said output winding when said field winding is supplied withcurrent; a rectifier means having alternating current input terminalsconnected to said output winding and first and second direct currentpower output terminals, said first power output terminal supplying thedesired value of regulated voltage to the electrical loads of said motorvehicle with said second power output terminal being connected to saidreference potential; an output transistor operative between conductiveand nonconductive states including emitter, collector and baseelectrodes; means connecting the collector-emitter circuit of saidoutput transistor in a series circuit connection with said field windingand across said output winding for supplying current to said fieldwinding when said output transistor is conductive; a voltage sensingcircuit connected to said first and second power output terminals; meansconnecting said voltage sensing circuit to the base of said outputtransistor for developing a voltage having a first predetermined valuewhich is operative to bias said output transistor conductive when thevoltage across said first and second power output terminals is belowsaid desired regulated value and having a second predetermined valuewhich is operative to bias said output transistor nonconductive when thevoltage across the first and second power output terminals is above saiddesired regulated value; a plurality of controlled rectifiers includinganode, cathode and gate electrodes, means connecting the anode andcathode of each of said plurality of controlled rectifiers between saidoutput winding and said reference potential so that said voltageproduced in said output winding is shunted to said reference potentialwhen said plurality of controlled rectifiers are gates conductive; acontrol transistor including collector, emitter and base electrodes,means connecting the collector-emitter circuit of said controltransistor to the base of said output transistor to bias said outputtransistor nonconductive when said control transistor is conductivethereby deenergizing the current supply to said field winding; a timedelay network including a capacitor and a resistor having apredetermined time constant connected to the base-emitter circuit ofsaid control transistor; a single controlled rectifier including anode,cathode and gate electrodes; means connecting the anode and cathode ofsaid single controlled rectifier in a series circuit with said timedelay network and across said first and second power output terminalsfor charging said capacitor from the voltage occuring thereacross whensaid single controlled rectifier is gated conductive, the voltagedeveloped across said capacitor being discharged through said resistorand the base-emitter circuit of said control transistor to bias saidcontrol transistor conductive for a predetermined time interval; and anovervoltage sensing device connected between said first power outputterminal and each of said gate electrodes of said plurality of saidcontrolled rectifiers and said single control rectifier, saidovervoltage sensing device being rendered conductive in response to apredetermined level of a high transient voltage developed in said outputwinding and across said first and second power output terminals so thatsaid plurality of controlled rectifiers and said single controlledrectifier are gated conductive, whereby said high transient voltage isshunted to said reference potential and concurrently said field windingis deenergized for said predetermined time interval.

2. A motor vehicle electrical generating system for supplying directcurrent power at a voltage having a desired regulated value relative toa reference potential, said system comprising: an alternating currentgenerator including a three phase output winding and a field windingwhich produces three-phase altemating-current voltage in said outputwinding when said field winding is supplied with current; a powerrectifier circuit means having alternating current input terminalsconnected to each phase of said output winding and positive and negativeoutput terminals, said positive output terminal supplying said directcurrent power at said desired regulated value of voltage said negativeoutput terminal being connected to said reference potential; a directcurrent electrical load of said motor vehicle; a conductor meansconnecting said electrical load across said positive and negative outputterminals for supplying said direct current power to said load; anoutput transistor including emitter, collector, and base electrodesoperative between conductive and nonconductive states; means connectingthe collector-emitter circuit of said output transistor in a seriesconnection with said field winding and across said positive and negativeoutput terminals for supplying current to said field winding when saidoutput transistor is conductive; a voltage sensing circuit connected tosaid positive and said negative output terminals; means connecting saidvoltage sensing circuit to the base of said output transistor fordeveloping a voltage having a first predetennined value which isoperative to bias said output transistor conductive when the voltageacross said positive and negative output terminals is below said desiredregulated value and having a second predetermined value which isoperative to bias said output transistor nonconductive when the voltageacross said positive and negative output terminals is above said desiredregulated value; a group of three controlled rectifiers including anode,cathode and gate electrode; means connecting the anode and cathodeofeach of said three controlled rectifiers betweenone phase of saidoutput'winding and said reference'potential so that when said group ofcontrolled rectifiers is conductive positively poled current paths areprovided from said output winding to said reference potential; a zenerdiode having a predetermined breakdown voltage connected between saidpositive output terminal and each of said gates of said group ofcontrolled rectifiers; said zener diode gating said group of controlledrectifiers conductive when said predetermined breakdown voltage isdeveloped thereacross by a high transient voltage generated in saidoutput winding by sudden disconnection of said electrical load; acontrol transistor including collector, emitter and base electrodes,means connecting the collectoremitter circuit of said control transistorto the base of said output transistor to bias said output transistornonconductive when said control'transistor is conductive therebydeenergizing the current supply to said field winding; a time delaynetwork including a capacitor and a resistor connected together andacross the base-emitter circuit of said control rectifier and having apredetermined time constant; a single controlled rectifier includinganode, cathode and gate electrodes; a coupling capacitor means connectedbetween said zener diode and said gate of said single controlledrectifier whereby said single controlled rectifier is gated conductiveby a triggering pulse developed across said coupling capacitor meanswhen said predetermined breakdown voltage is developed across said zenerdiode and being maintained conductive thereafter by forward poledvoltage applied across the anode and cathode thereof; means connectingsaid anode and cathode of said single controlled rectifier in serieswith said capacitor of said time delay network and across said positiveand negative power output terminals for charging said capacitor from thevoltage developed across said output terminals when said singlecontrolled rectifier is gated conductive, the voltage developed acrosssaid capacitor rendering said single controlled rectifier nonconductiveby opposing the forward poled voltage and being discharged from saidcapacitor through a circuit path including said resistor andthebase-emitter circuit of said control transistor thereby biasing saidcontrol transistor conductive for a predetermined'time intervalcorresponding to the time constant to said time delay network, wherebysaid high transient voltage is shunted from said output winding to saidreference potential and concurrently said field winding is deenergizedfor said predetermined time interval.

1. A motor vehicle electrical generating system for supplying directcurrent power at a voltage having a desired regulated value relative toa reference potential, said system comprising: an alternating currentgenerator including an output winding and a field winding which producesvoltage in said output winding when said field winding is supplied withcurrent; a rectifier means having alternating current input terminalsconnected to said output winding and first and second direct currentpower output terminals, said first power output terminal supplying thedesired value of regulated voltage to the electrical loads of said motorvehicle with said second power output terminal being connected to saidreference potential; an output transistor operative between conductiveand nonconductive states including emitter, collector and baseelectrodes; means connecting the collector-emitter circuit of saidoutput transistor in a series circuit connection with said field windingand across said output winding for supplying current to said fieldwinding when said output transistor is conductive; a voltage sensingcircuit connected to said first and second power output terminals; meansconnecting said voltage sensing circuit to the base of said outputtransistor for developing a voltage having a first predetermined valuewhich is operative to bias said output transistor conductive when thevoltage across said first and second power output terminals is belowsaid desired regulated value and having a second predetermined valuewhich is operative to bias said output transistor nonconductive when thevoltage across the first and second power output terminals is above saiddesired regulated value; a plurality of controlled rectifiers includinganode, cathode and gate electrodes, means connecting the anode andcathode of each of said plurality of controlled rectifiers between saidoutput winding and said reference potential so that said voltageproduced in said output winding is shunted to said reference potentialwhen said plurality of controlled rectifiers are gates conductive; acontrol transistor including collector, emitter and base electrodes,means connecting the collector-emitter circuit of said controltransistor to the base of said output transistor to bias said outputtransistor nonconductive when said control transistor is conductivethereby deenergizing the current supply to said field winding; a timedelay network inclUding a capacitor and a resistor having apredetermined time constant connected to the base-emitter circuit ofsaid control transistor; a single controlled rectifier including anode,cathode and gate electrodes; means connecting the anode and cathode ofsaid single controlled rectifier in a series circuit with said timedelay network and across said first and second power output terminalsfor charging said capacitor from the voltage occuring thereacross whensaid single controlled rectifier is gated conductive, the voltagedeveloped across said capacitor being discharged through said resistorand the base-emitter circuit of said control transistor to bias saidcontrol transistor conductive for a predetermined time interval; and anovervoltage sensing device connected between said first power outputterminal and each of said gate electrodes of said plurality of saidcontrolled rectifiers and said single control rectifier, saidovervoltage sensing device being rendered conductive in response to apredetermined level of a high transient voltage developed in said outputwinding and across said first and second power output terminals so thatsaid plurality of controlled rectifiers and said single controlledrectifier are gated conductive, whereby said high transient voltage isshunted to said reference potential and concurrently said field windingis deenergized for said predetermined time interval.
 2. A motor vehicleelectrical generating system for supplying direct current power at avoltage having a desired regulated value relative to a referencepotential, said system comprising: an alternating current generatorincluding a three-phase output winding and a field winding whichproduces three-phase alternating-current voltage in said output windingwhen said field winding is supplied with current; a power rectifiercircuit means having alternating current input terminals connected toeach phase of said output winding and positive and negative outputterminals, said positive output terminal supplying said direct currentpower at said desired regulated value of voltage said negative outputterminal being connected to said reference potential; a direct currentelectrical load of said motor vehicle; a conductor means connecting saidelectrical load across said positive and negative output terminals forsupplying said direct current power to said load; an output transistorincluding emitter, collector, and base electrodes operative betweenconductive and nonconductive states; means connecting thecollector-emitter circuit of said output transistor in a seriesconnection with said field winding and across said positive and negativeoutput terminals for supplying current to said field winding when saidoutput transistor is conductive; a voltage sensing circuit connected tosaid positive and said negative output terminals; means connecting saidvoltage sensing circuit to the base of said output transistor fordeveloping a voltage having a first predetermined value which isoperative to bias said output transistor conductive when the voltageacross said positive and negative output terminals is below said desiredregulated value and having a second predetermined value which isoperative to bias said output transistor nonconductive when the voltageacross said positive and negative output terminals is above said desiredregulated value; a group of three controlled rectifiers including anode,cathode and gate electrode; means connecting the anode and cathode ofeach of said three controlled rectifiers between one phase of saidoutput winding and said reference potential so that when said group ofcontrolled rectifiers is conductive positively poled current paths areprovided from said output winding to said reference potential; a zenerdiode having a predetermined breakdown voltage connected between saidpositive output terminal and each of said gates of said group ofcontrolled rectifiers, said zener diode gating said group of controlledrectifiers conductive when said predetermined breakdoWn voltage isdeveloped thereacross by a high transient voltage generated in saidoutput winding by sudden disconnection of said electrical load; acontrol transistor including collector, emitter and base electrodes,means connecting the collector-emitter circuit of said controltransistor to the base of said output transistor to bias said outputtransistor nonconductive when said control transistor is conductivethereby deenergizing the current supply to said field winding; a timedelay network including a capacitor and a resistor connected togetherand across the base-emitter circuit of said control rectifier and havinga predetermined time constant; a single controlled rectifier includinganode, cathode and gate electrodes; a coupling capacitor means connectedbetween said zener diode and said gate of said single controlledrectifier whereby said single controlled rectifier is gated conductiveby a triggering pulse developed across said coupling capacitor meanswhen said predetermined breakdown voltage is developed across said zenerdiode and being maintained conductive thereafter by forward poledvoltage applied across the anode and cathode thereof; means connectingsaid anode and cathode of said single controlled rectifier in serieswith said capacitor of said time delay network and across said positiveand negative power output terminals for charging said capacitor from thevoltage developed across said output terminals when said singlecontrolled rectifier is gated conductive, the voltage developed acrosssaid capacitor rendering said single controlled rectifier nonconductiveby opposing the forward poled voltage and being discharged from saidcapacitor through a circuit path including said resistor and thebase-emitter circuit of said control transistor thereby biasing saidcontrol transistor conductive for a predetermined time intervalcorresponding to the time constant to said time delay network, wherebysaid high transient voltage is shunted from said output winding to saidreference potential and concurrently said field winding is deenergizedfor said predetermined time interval.